| To measure the viscosity of densifying materials as a function of both porosity and temperature, isothermal cyclic loading dilatometry (ICLD) is proposed as a convenient technique. We demonstrate its merit relative to constant load techniques in minimizing the stress history effects (changes in shrinkage anisotropy and sample microstructure) that arise due to the application of an external load. A constant load test overestimates the viscosity by an order of magnitude compared to a cyclic load test. To obtain accurate viscosity data, maximum loading rates and longer unloading periods are desirable as they reduce effects of shrinkage anisotropy on viscosity values.; A novel technique, based on the concept of pressureless constrained sintering and the viscous analogy, for determining the viscous Poisson's ratio of sintering materials is proposed. The method involves measuring the sintering rate of a free-sintered specimen, and a specimen constrained by two non-sintering layers.; Predicting the sintering viscosity accurately is key to developing reliable sintering models. A theoretical understanding for the observed viscosity of LTCC materials, and a framework for predicting the evolution of their viscous behavior during sintering was developed. The complex viscosity of these materials is affected by a host of interdependent factors such as the base glass composition, temperature, porosity, particle size and distribution, contact area, volume fraction of filler particles, phase separation, crystallization and heating rate. A model based on the simplifying assumption that these variables are mutually exclusive is presented. The model predictions were in reasonable agreement with measured data.; The concept of constrained sintering has attracted a lot of attention as it offers the advantages of tight dimensional tolerances and minimal distortion. The effect of an external constraint (uniaxial compression and pressure-less constraint) on the microstructure, density and shrinkage anisotropy during the sintering of CT and DU were studied. The difference in the viscous behavior of the two materials led to significantly different microstructures, density and shrinkage anisotropy. The shrinkage anisotropy constant of DU was significantly higher than that of CT and non-linear under uniaxial compression. A combination of higher viscosity, lower viscous Poisson's ratio, and higher constraining stresses led to lower densities for CT compared to DU. The maximum tensile stress due to a pressure-less constraint, assuming the viscous model, was in the range of ∼10--100 kPa. The calculated uniaxial compressive stress required for zero radial shrinkage in the perpendicular plane was in the range of ∼10--180 kPa. (Abstract shortened by UMI.)... |
